Seasonal Progression, Symptom Development, and Yield Effects of Alfalfa mosaic virus Epidemics on Soybean in Wisconsin.

The occurrence of Alfalfa mosaic virus (AMV) has increased in Wisconsin soybean fields in conjunction with the establishment of the soybean aphid (Aphis glycines). Field studies were conducted to determine the seasonal incidence of AMV-infected plants, progression of symptom severity caused by AMV, and the effect of AMV on soybean yield and seed quality. An isolate of AMV, collected from field-grown soybean, was introduced into plots by mechanical inoculation of plants at the V1 growth stage. The goal of the experiment was to achieve an incidence of AMV-infected plants of 0, 50, and 100% in 2002, and 0, 10, 25, 50, 75, and 100% in 2003. Severity of foliar symptoms was rated, and leaf samples were collected for serological assay (enzyme-linked immunosorbent assay [ELISA]) to estimate the incidence of AMV-infected plants from growth stages VC to R5. The maximum incidence of AMV-infected plants was 49% at growth stage R5, yet detection of the virus by ELISA dropped dramatically thereafter in both years. Incidence of AMV-infected plants accounted for 53 to 67% of the variability observed for severity of foliar symptoms in 2002 and 2003, respectively. Maximum yield loss ranged from 32% in 2002 to 48% in 2003 based on the difference in yield between noninoculated plots and plots with the highest incidence of AMV-infected plants. Incidence of AMV-infected plants explained 31% of the variation in yield in 2002 and 26% in 2003. An AMV incidence of 30% or greater was required for yield loss caused by AMV. Results of this study suggest that AMV has the potential to lower soybean yield and warrants further study.